It looks like even if you’re not storing the backups locally, you need to have a valid, local backup path or the plugin will not back up.
I was having the same error about “some other process” while trying to manually back up. Once I edited the local address to a real, writable directory, I can now back up.
This is definitely a defect in the plugin.
Good luck!
I’m not getting any errors as well. Using WordPress SEO 1.4.22. Custom Contact Forms 5.1.0.3.